Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX), and meta-xylene (MX) are often present together in C8 aromatic streams from chemical plants and oil refineries. Although EB is an important raw material for the production of styrene, for a variety of reasons, most EB feedstocks used in styrene production are produced by alkylation of benzene with ethylene, rather than by recovery from a C8 aromatics stream. Of the three xylene isomers, PX has the largest commercial market and is used primarily for manufacturing terephthalic acid and terephthalate esters for use in the production of various polymers such as poly(ethylene terephthalate), poly(propylene terephthalate), and poly(butene terephthalate). While OX and MX are useful as solvents and raw materials for making products such as phthalic anhydride and isophthalic acid, market demand for OX and MX and their downstream derivatives is much smaller than that for PX.
Given the higher demand for PX as compared with its other isomers, there is significant commercial interest in maximizing PX production from any given source of C8 aromatic materials. However, there are two major technical challenges in achieving this goal of maximizing PX yield. Firstly, the C8 aromatics are difficult to separate due to their similar chemical structures, physical properties, and identical molecular weights. Secondly, the four C8 aromatic compounds, particularly the three xylene isomers, are usually present in concentrations dictated by the thermodynamic equilibriums of the C8 aromatics. Under the conditions of 200° C. to 500° C. at which xylenes are processed in typical petrochemical plants, the thermodynamic equilibrium content calculated based on free energy of formation is often approximately 24 wt % PX, 56 wt % MX, and 20 wt % OX, based on the total amount of xylenes in the feed. Such a relatively low PX equilibrium concentration leads to large amounts of MX and OX recycles which are reprocessed through several energy intensive operations, making PX production a costly practice in terms of energy consumption and capital investments. Present demand for PX is fairly large and is expected to grow in the future. Consequently, a system maximizing PX production in an energy-efficient manner is highly sought after.
A typical xylenes production process is illustrated in FIG. 1. The feed streams to the system comprise C8+ aromatics and may come from one or more sources, including C8+ reformate 1 (see, for instance, U.S. Pat. No. 7,179,367), C8+ selective toluene disproportionation product 17 (see, for instance, U.S. Pat. No. 7,989,672), C8+ transalkylation product 2 (see, for instance, U.S. Pat. No. 7,663,010), C8+ toluene disproportionation product 15 (see, for instance, U.S. Pat. No. 6,198,013), and C8 aromatics, produced from toluene and/or benzene methylation with methanol (see, for instance, U.S. Application 2011/0092755). These streams typically comprise C8 and heavier aromatics which are processed along with a recycle stream 10 in one or more fractionators 16 for the removal C9+ aromatics (aromatic compounds having nine or more carbon atoms) and, optionally, OX in stream 3, which, optionally, can be subsequently separated in fractionator 14 into OX overhead 4 and C9+ bottoms 5. The C9+ aromatics could have adverse effects on downstream PX Recovery 12 and vapor-phase xylenes isomerization unit 13 if not removed from the feed stream(s) as bottoms by the aforementioned fractionation unit 16 and, optionally, 14.
The removal of C9+ aromatics and, optionally, OX in fractionator(s) 16 thus yields an overhead C8 aromatics-rich stream 6 which typically contains PX at a concentration of below or near the thermodynamic equilibrium concentration. The C8-aromatics-rich stream 6 is processed to selectively recover PX in a xylenes separation system shown as PX recovery 12 which may be one or both of selective adsorption or crystallization. A PX-rich product stream 7, typically having more than 99.7 wt % PX is recovered, and a PX-depleted raffinate stream 8 containing the balance of C8 aromatics stream passes to vapor-phase xylenes isomerization 13. Usually, in the presence of hydrogen in stream 9, vapor-phase xylenes isomerization 13 generates an isomerate (i.e., isomerization product) stream 19 having near-equilibrium concentration of xylene isomers using one or more of a variety of catalysts which may also convert EB to benzene and ethane or may convert EB to near-equilibrium xylene isomers. The isomerate, or isomerization product stream 19 passes to detoluenization fractionation 18 which removes C7− hydrocarbons (hydrocarbon compounds having seven or less carbon atoms) in stream 11 to yield isomerate recycle stream 10. Isomerate recycle stream 10 is processed in fractionator 16.
The above processing steps, including fractionator 16, PX recovery 12, vapor-phase xylenes isomerization 13, and detoluenization fractionation 18, are all energy-intensive operations. As shown in FIG. 1, conventional xylenes production processes normally involve recycling a stream between the separations, in which most of the PX is recovered and a PX-depleted raffinate stream is produced, and the isomerization, in which the PX content of the PX-depleted raffinate stream is returned back towards equilibrium concentration. However, these processes suffer from the deficiencies that (1) the low PX concentration in the feed to PX recovery 12 leads to the large quantity of recycle stream 10 and (2) recycle stream 10 must be reprocessed through all the energy-intensive steps. Such deficiencies make conventional xylenes production a costly operation in terms of both capital and energy.
Improving such energy-intensive processes is an active area of research, but it is not a simple matter of optimization of each individual step, as optimization of one step may negatively affect one or more steps in the overall system. Examples of proposed improvements include the following.
U.S. Pat. No. 7,439,412 discloses a process for recovering one or more high purity xylene isomers from a C8+-aromatic feed stream including the use of an isomerization unit under liquid-phase conditions. In an example, the product of the liquid-phase isomerization unit is returned to the first fractionation tower in the system.
U.S. Pat. No. 7,553,998 discloses a process for recovering one or more high-purity xylene isomers from a feed having substantial content of C9+ aromatic hydrocarbons comprising de-ethylation of heavy aromatics followed by fractionation and then passing the stream to a C8-aromatic-isomer recovery to recover high-purity xylene isomers with lowered energy costs. Streams passing through an isomerization unit under liquid isomerization conditions are split, with a portion sent to an isomer recovery unit, and a portion is purged.
U.S. Pat. No. 7,626,065 discloses processes for recovering one or more high-purity xylene isomers from a feed having substantial content of C9+ aromatic hydrocarbons comprising using an additional xylenes separator to generate a PX-rich effluent stream, which serves as the feed to PX recovery, and a PX-depleted effluent stream, which is converted to near equilibrium using an additional xylenes isomerization. The arrangements save energy by reducing the amount of isomerate recycle.
WO 2012/058106 and WO 2012/058108 describe processes for producing a PX-rich product, such as (a) providing a PX-depleted raffinate stream; (b) providing a parallel configuration of vapor-phase and liquid-phase isomerization units; and (c) splitting the PX-depleted raffinate stream and isomerizing the two split streams in the two parallel isomerization units respectively. The process saves energy by reducing the amount of isomerate recycle from vapor-phase xylenes isomerization which is more energy intensive than liquid-phase xylenes isomerization.
WO 2011/133326 is directed to a xylenes isomerization process, including a liquid-phase isomerization, for the production of equilibrium or near-equilibrium xylenes, wherein the process conditions include a temperature of less than 295° C. and a pressure sufficient to maintain the xylenes in liquid phase that uses at most only ppm levels of hydrogen and that in embodiments can be regenerated numerous times by an in situ procedure.
Other references of interest include U.S. Publication Nos. 2008/0262282; 2009/0149686; 2009/0182182; U.S. Pat. Nos. 6,448,459; 6,872,866; and 7,368,620.
Present demand for PX is fairly large and is expected to grow in the future. Consequently, a system maximizing PX production in an energy-efficient manner is highly sought after. While prior attempts to improve PX and, optionally, OX production abound, most have not been able to reduce the xylenes recycle and circumvent the energy intensive vapor-phase isomerization unit simultaneously. The present inventors have surprisingly discovered processes which reduce xylenes recycle and avoid vapor-phase isomerization to further lower energy consumption by coupling two in-series xylenes separation systems with two parallel isomerization systems. The improved processes significantly reduce the energy required and/or increase the production capacity for producing high purity PX and, optionally, OX.